Radio frequency power amplifiers (RFPAs) are used in a wide variety of applications. As illustrated in FIG. 1, an RFPA 100 operates to convert an RF input signal, RFin, having a small amount of energy to an amplified RF output signal, RFout, having a large amount of energy. The energy required to complete this conversion process is provided by a direct current (DC) voltage supply, Vsupply, typically a battery.
The approach to supplying power to the RFPA 100 in FIG. 1 is known as a “fixed drain bias” approach, since the drain of the transistor (typically, a field-effect transistor having a gate, a drain and a source) in the RFPA 100 is directly coupled to the fixed DC voltage supply, Vsupply. Unfortunately, an RFPA powered by a fixed drain bias is not a very efficient converter of power. In fact, the RFPA 100 becomes progressively less efficient the smaller the amplitude of the RF input signal, RFin, is compared to the fixed DC voltage supply, Vsupply. Accordingly, in many applications, and in particular those that use non-constant-envelope signals, the fixed drain bias approach is quite often not a suitable approach to delivering power to an RFPA.
One technique that can be used to improve the efficiency over the efficiencies obtainable can be obtained using a fixed drain bias, is to use what is known as an “envelope tracking” (ET) system, a simplified drawing of which is shown FIG. 2. The ET system 200 comprises an envelope modulator 202 and an RFPA 204. The envelope modulator 202 operates to modulate the power supply voltage, Vsupply, according to an envelope signal, Venv, containing envelope information of the RF input signal, RFin, applied to the RFPA 204. The resulting envelope modulated power supply signal, Vout, is coupled to a power supply input of the RFPA 204. The RFPA 204 then amplifies the RF input signal, RFin, according to the envelope modulated power supply signal, Vout, thereby providing the desired RF output signal, RFout. Because the envelope modulated power supply signal, Vout, tracks the envelope of the RF input signal, RFin, the RFPA 204 is able to operate more efficiently compared to an RFPA with a fixed drain bias.
The envelope modulator 202 in the ET system 200 in FIG. 2 can be implemented in various ways. One approach is to use a linear regulator. A linear regulator, as the name suggests, generates an output signal that is linearly related to the signal applied to its input. Accordingly, when an envelope signal, Venv, is applied to the input of the linear regulator, as shown in FIG. 3, the linear regulator provides an envelope modulated power supply signal, Vout, which linearly tracks the amplitude variations of the envelope signal, Venv.
One attractive characteristic of the linear regulator 300 is that it can react quickly to sudden changes in the envelope signal, Venv. Hence, if used to implement the envelope modulator 202 in the ET system 200 in FIG. 2, the ET system 200 is afforded the ability to operate over a wide bandwidth. Wide bandwidth operation is highly desirable, since many modern communications systems such as, for example, Orthogonal Frequency-Division Multiplexing-based (OFDM-based) 802.11a/g wireless local area networks (or “Wi-Fi” LANs) and Wideband Code Division Multiple Access (W-CDMA) cellular communications systems, use wideband signaling. One significant drawback of the linear regulator, however, is that it is inefficient for input signal amplitudes that are lower than the magnitude of the DC supply voltage, Vsupply. The inefficiency increases as this voltage difference widens.
An alternative converting device, which can be used to implement the envelope modulator 202 in the ET system 200 in FIG. 2, and which is much more efficient than the linear regulator 300 in FIG. 3, is a switch-mode converter. FIG. 4 is a diagram of a typical switch-mode converter 400 (also referred to in the art as a “step-down” converter or a “buck” converter). The switch-mode converter 400 includes a power (or “switching”) transistor 402 configured to operate as a switch, an inductor 404, and a capacitor 406. The switching transistor 402 is controlled by a pulse-width modulated switch control signal provided by a comparator 408, which is configured to operate as a pulse-width modulator. The pulse-width modulated switch control signal is a square wave having a duty cycle, D. The duty cycle, D, varies according to changes in the amplitude of the envelope signal, Venv. When the pulse-width modulated switch control signal is applied to the gate of the switching transistor 402, it turns the switching transistor 402 on and off, thereby alternately connecting and disconnecting the inductor 404 to and from the DC supply voltage, Vsupply. The inductor 404 and capacitor 406 operate as a low-pass filter, to filter the inductor current before it is transferred to the load 410. It can be shown that the resulting output voltage signal, Vout, is proportional to the product of the duty cycle, D, and the magnitude of the DC supply voltage, Vsupply. In other words, the resulting output voltage signal, Vout, is an envelope modulated power supply signal that tracks the amplitude variations of the envelope signal, Venv.
While the switch-mode converter 400 in FIG. 4 is capable of generating an envelope modulated power supply signal efficiently, it is slow and noisy. It is noisy due to the switching action of the switching transistor 402. Filtering cannot completely remove the switching noise, and inevitably some amount of switching noise is introduced into the RF output signal, RFout, of the RFPA. This switching noise makes it difficult to satisfy signal to noise ratio requirements required of wireless standards. The switch-mode converter 400 is slow due to the large gate capacitance presented by the large switching transistor. Generating and sourcing large currents requires a transistor with a large gate area. However, a large gate area introduces a large parasitic capacitance (on the order of 1000 pF), which limits the switching speed of the switching transistor 402 to only about 5 MHz or so. Given that accurate envelope tracking requires a switching frequency of twenty to fifty times higher than the required signal envelope bandwidth, and many signal types have a signal envelope bandwidth of 1 MHz or higher, switch-mode converters are not typically well-suited for wide bandwidth ET tracking.
Given the need for an ET system that is both efficient and capable of operating over a wide bandwidth, various techniques have been proposed which combine the high bandwidth and low-noise capabilities of the linear regulator with the high-efficiency capability of the switch-mode converter. FIG. 5 is a drawing of an ET system 500 of one such approach. The ET system 500 comprises an envelope modulator 502, and an RFPA 504. The envelope modulator 502 comprises a linear regulator 506 (similar to the linear regulator 300 shown and described above in connection with FIG. 3), a hysteresis comparator 508, and a buck converter 510 (similar to the buck converter shown and described above in connection with FIG. 4). The hysteresis comparator 508 operates to provide a switch control signal to the switching transistor 512 of the buck converter 510, based on the direction of current flow sensed by a current sense resistor 514. The direction of current flow is determined by whether the linear regulator 506 is sourcing current to the RFPA 504 or is sinking excess current supplied from the buck converter 510. When the buck converter 510 provides too much current required of the RFPA 504, any excess current not needed by the RFPA 504 is sunk by the linear regulator 506. At times when the required instantaneous current required of the RFPA 504 becomes greater than the instantaneous switch current being supplied by the buck converter 510, the extra current needed by the RFPA 504 is sourced to the RFPA 504 by the linear regulator 506.
While the ET system 500 in FIG. 5 is capable of operating over a wide bandwidth, it is less efficient than desired. It would be desirable, therefore, to have ET systems and methods that are capable of tracking wide bandwidth signals and which are more efficient than the ET systems and methods available in the prior art.